1. Field of the Invention
This invention directs itself to a system and method of mitigating the effects of signal fades, temporary blockages or severe channel impairments in an audio broadcasting system. More particularly, the system and method employs the transmission of a primary broadcast signal along with a redundant signal, the redundant signal being delayed by a predetermined amount of time, on the order of several seconds, with respect to the primary broadcast signal. A corresponding delay is incorporated in the receiver for delaying the received primary broadcast signal. Still further, this invention is directed to the concept of detecting degradation in the primary broadcast channel that represents a fade or blockage in the RF signal, before such is perceived by the listener. In response to such detection, the delayed redundant signal is temporarily substituted for the corrupted primary audio signal, acting as a “gap filler” when the primary signal is corrupted or unavailable. More in particular, this invention directs itself to use of a blend function for smoothly transitioning from the primary audio signal to the delayed redundant signal.
2. Prior Art
In fixed receiver installations, such as home receivers, the fading statistics are generally stationary, except for occasional temporary fades caused by passing vehicles or aircraft, and so effective mitigation of fades and blockages for these applications can be as simple as installing a better antenna or repositioning the existing antenna. In automotive applications, however, fading and blockage statistics are not stationary, being dependent on the vehicle location and velocity, and effective mitigation requires more sophisticated methods.
Digital Audio Broadcasting (DAB) techniques are being proposed to improve the quality of broadcasting over conventional AM and FM analog signals. In-Band-On-Channel (IBOC) DAB is a digital broadcasting scheme, likely to be adopted in the United States, in which analog AM or FM signals are simulcast along with the DAB signal The digital audio signal is generally compressed such that a minimum data rate is required to convey the audio information with sufficiently high fidelity. Terrestrial DAB systems generally have the characteristic that fades and blockages have a more deleterious effect on received audio than they do on analog modulated systems, such as commercial AM or FM broadcasts because these DAB systems do not degrade gracefully. This effect is exacerbated for in-band on-channel (IBOC) systems which are constrained to have orders of magnitude lower transmit power than the analog broadcast signals whose frequency band they share. IBOC DAB systems transmit both the analog and DAB signal simultaneously within the required spectral mask of the analog signal alone. Therefore, the IBOC DAB concept allows a station to offer digital audio while retaining its analog listeners, but the digital broadcast will not gain acceptance unless the audio loss due to temporary fades and blockages is mitigated.